Natural explanations include an orbiting planet or comet, but the dimming is too irregular, leaving some to suggest alien activity.

The most controversial star in the universe just got even more extraordinary. Astronomers have zeroed in on a strange star named KIC 8462852 (also known as Tabby's star) that has been experiencing odd dimming patterns. And after a few months without any dimming, recently a dimming event of 2 percent was recorded in just one night — which may not sound like much, but it's actually quite significant. Now scientists around the world are watching closely, hoping to discover why the star dims.

Usually when astronomers witness a faint dim in a distant star, this is due to an orbiting planet passing in front of it — nothing too unusual. KIC 8462852's dimming pattern is unlike any other, however. It is irregular and very pronounced, indicating that a large structure or whole jumble of objects in tight formation must be orbiting the star. This sort of pattern has never been seen before, which has led to some fanciful theories. For instance, some theorists have even suggested the possibility of intelligent design, that perhaps the jumble of objects represent a megastructure built by aliens to harness the power of the star.

Of course, this is mere speculation. By contrast, the leading natural explanation proposed so far theorizes that the debris field passing in front of the star could be a sea of comets that were recently yanked inward into the solar system by the gravity of another close-passing star.

The comet theory had been gaining momentum, but now evidence has emerged that appears to discredit it, reports New Scientist. In other words, astronomers are back to square one. And yes, that means the alien theory is still on the table.

A long history of dimming

The evidence comes from Bradley Schaefer of Louisiana State University, who performed a historical analysis of digitally scanned photographic plates of the sky since the 1890s that included KIC 8462852. It turns out that odd dimming patterns are nothing new for this strange star. Schaefer found that KIC 8462852 had also dimmed by about 20 percent between 1890 and 1989.

In order for comets to explain these patterns, it would require about 648,000 comets, each 200 kilometers wide, to have passed in front of the star. The odds of this happening are staggering, to say the least.

“The comet-family idea was reasonably put forth as the best of the proposals, even while acknowledging that they all were a poor lot,” said Schaefer. “But now we have a refutation of the idea, and indeed, of all published ideas.”

So if the best natural explanation is now refuted, does this mean the alien megastructure idea is more likely? Well, not really. But the alien theory is certainly one that won't be put to rest very easily, especially given that scientists are at a loss to explain the dimming pattern given known natural phenomenon.

Schaefer himself remains skeptical of the alien theory. “The alien-megastructure idea runs wrong with my new observations,” he said, noting that it would require a monumental technological undertaking to build a structure capable of covering a fifth of a star in a mere century.

Even so, if we're willing to grant that it's possible for aliens to build such a structure at all, why couldn't they also be advanced enough to build it within a time frame of 100 years? Given the absence of other reasonable theories, almost anything is possible at this point.

“Either one of our refutations has some hidden loophole, or some theorist needs to come up with some other proposal,” said Schaefer.

Spurred on by the controversy surrounding Schaefer's numbers, astronomers Ben Montet from Caltech and Joshua Simon of the Carnegie Institute decided to take a longer look at Kepler's observations of KIC 8462852. "We realized that in order to settle this, you needed either a long baseline, or high precision data," Montet told Gizmodo.

In August 2016, Montet and Simon shared a non-peer reviewed paper that did exactly that. The pair of researchers examined Kepler's four years' worth of data on KIC 8462852 and found that over that period, the star dimmed by around 3 percent. For the first 1,000 days that Kepler eyed the star, its luminosity dipped by 0.34 percent per year. The following 200 days saw significant drop off in brightness of 2 percent before KIC 8462852 seem to stabilize.

"The part that really surprised me was just how rapid and non-linear it was," Montet said to Gizmodo. "We spent a long time trying to convince ourselves this wasn't real. We just weren’t able to."

Montet and Simon checked 500 stars in proximity to KIC 8462852 and then another 500 stars that resemble it in terms of size and composition to see if their brightness reduced along similar lines over the same time periods, but none of them did.

As to what may be responsible for the dimming, Montet isn't sure. Whether it's some weird alien structure, a swarm of comets or something else entirely, he says that "nothing nicely explains everything."

Editor's note: This story was originally published in January 2016 and has been updated with new information.